"It is known to be an active fault, but it certainly has not been viewed as one of the major players in the Bay Area," Brocher said.
Napa's 6.0-magnitude earthquake is the biggest quake in California since the magnitude-6.7 Northridge earthquake in 1994, and the biggest in Northern California since the 1989 Loma Prieta earthquake, a magnitude 6.9.
Collecting clues But the South Napa earthquake could have been worse. Only part of the West Napa Fault broke open. If the entire fault had ripped, the earthquake would have been more powerful.
"I have to say, as much as the earthquake was large and had a widespread impact, it's not as bad as it could have been," Mark Ghilarducci, director of California's Office of Emergency Services, said during a press conference Sunday.
Though the actual earthquake rupture only lasted four seconds, there was strong shaking in Napa because the valley floor is lined with soft river soils that amplify earthquake waves. Also, the fault zipped open from south to north, directing the earthquake energy toward the city. "It looks like it went northwest and up from the dot on the map that shows where the earthquake nucleated," Brocher said. [13 Crazy Earthquake Facts]